This invention relates to a mask blank for use in producing an integrated circuit such as a semiconductor and an optical component having an optical function provided by a fine or microscopic pattern, a method of manufacturing an exposure mask, and a method of manufacturing an imprint template.
Generally, in a manufacturing process of a semiconductor device or the like, a fine pattern is formed by the use of photolithography. In order to form the fine pattern, a plurality of substrates called photomasks are used. The photomask generally includes a light-transmitting glass substrate and a light-shielding fine pattern formed on the substrate and comprising a metal thin film or the like. In manufacture of the photomask, photolithography is used also.
The above-mentioned photomask or an imprint template is used as a master for transferring the same fine pattern onto a large number of objects. The dimensional accuracy of the pattern formed on the photomask directly affects the dimensional accuracy of the fine pattern to be produced by pattern transfer. In case of the imprint template, a sectional shape of the pattern formed on the template also affects the shape of the fine pattern to be produced by pattern transfer. With the improvement in degree of integration of a semiconductor circuit, the dimension of the pattern is reduced and the photomask or the imprint template is required to have higher accuracy. Similarly, an optical component having an optical function provided by a fine pattern such as grating is also required to have a pattern dimension smaller than a target wavelength and pattern accuracy. Therefore, the photomask or the imprint template for use in production of the optical component is also required to have a microscopic and high-accuracy pattern.
In production of a conventional photomask and a conventional imprint template, use is made of a mask blank comprising a light-transmitting substrate, such as a quartz glass, and a thin film, such as a chromium film, formed on the substrate. After a resist is applied onto the mask blank, a resist pattern is formed by the use of EB (electron beam) exposure or the like. Using the resist pattern as a mask, the thin film is etched to thereby form a thin film pattern (mask pattern).
In order to further improve the resolution upon pattern transfer, the photomask may be produced as a phase shift mask. The phase shift mask is obtained by patterning, using the thin film pattern as a mask, the light-transmitting substrate or a light semi-transmitting film formed between the substrate and the thin film. In the imprint template, in order to irradiate light upon transfer, a step pattern may be formed on the light-transmitting substrate with the thin film pattern used as a mask. In either event, the dimensional accuracy of the light-transmitting substrate is directly affected by the dimensional accuracy of the thin film pattern.
For example, as means for etching the thin film containing chromium, use is generally made of wet etching using di-ammonium cerium (IV) nitrate or dry etching using a mixed gas containing a chlorine-based gas and oxygen.
Japanese Unexamined Patent Application Publication (JP-A) No. 2005-530338 (Patent Document 1) discloses a method in which a thin film pattern comprising a plurality of layers is formed by the use of multi-stage etching in order to avoid nonuniformity in etching width or depth of the chromium film. Japanese Unexamined Patent Application Publication (JP-A) No. 2006-78825 (Patent Document 2) discloses a method in which a relatively thin film pattern is formed with a resist pattern used as a mask and then, using the thin film pattern as a mask, thin film patterns of second and subsequent layers are formed. This method is intended to reduce the thickness of the resist.